1. Field of the Invention
This invention relates to an onium salt that generates an alkylimidic acid containing a specific fluorine group, a resist material for exposure to high-energy rays shorter than 300 nm in wavelength characterized by containing the onium salt, and a pattern forming method that uses the resist material.
2. Description of the Related Art
In recent years, integration and operating speeds have been further improved in LSI devices, and, correspondingly, miniaturization of a pattern rule has been sought. As such, far ultraviolet ray lithography and vacuum ultraviolet ray lithography are promising as next-generation microfabrication techniques.
Advanced semiconductors of a 0.15 μm rule are presently manufactured by photolithography using an KrF excimer laser, and, in addition, those of a 0.13 μm rule are about to come under production. Photolithography in which an ArF-excimer laser beam is used as a light source is regarded as a technique indispensable for hyperfine fabrication of not more than 0.13 μm, and it is greatly hoped that photolithography will be achieved.
Especially in photolithography in which the ArF excimer laser beam is used as a light source, a highly sensitive resist material capable of exhibiting sufficient resolution with less exposure is sought in order to prevent a precise, expensive optical material from deteriorating. The most common measure for realizing a highly sensitive resist material is to select highly transparent materials in a wavelength of 193 nm. For example, a poly(meth)acrylic acid and a derivative thereof, a norbornene-maleic anhydride alternating polymer, polynorbornene, and a metathesis polymer by ring-opening polymerization, etc., have been proposed with regard to a base resin, and, due to the fact that the transparency of a resin alone is heightened, a certain degree of success has been obtained. However, with regard to an acid generator, a rise in transparency causes a decrease in acid generation-efficiency, and, as a result, causes a low sensitivity or a lack of heat stability and shelf stability. Therefore, in the present state, a sufficiently practical acid generator has not-yet been obtained.
An alkylsulfonium salt, which is proposed in, for example, Japanese Patent Provisional Publication Nos. 7-25846/1995 (U.S. Pat. Nos. 5,585,507 and 5,635,332), 7-28237/1995 (U.S. Pat. Nos. 5,585,507 and 5,635,332), and 8-27102/1996, is not preferable because the salt has neither sufficient acid generation efficiency nor sufficient heat stability although the salt has very high transparency. An alkylarylsulfonium salt, which is proposed in Japanese Patent Provisional Publication No. 10-319581/1998, etc., is superior in balance between transparency and acid generation efficiency and is high in sensitivity, but lacks both heat stability and shelf stability. An arylsulfonium salt, which has been effective in photolithography using an KrF excimer laser beam, is superior in acid generation efficiency, in heat stability, and in shelf stability, but is extremely low in transparency, so that a pattern after being developed is severely tapered. Although there is a possible measure in which transparency is supplemented by thinning the film of a resist, this measure is not suitable as a pattern forming method because it markedly lowers the etching resistance of the resist film.
These publications chiefly describe a case where the structure on the cation side of the onium salt is changed, and it has been reported that, in resolution and pattern shape, a close relationship exists between the kind of generated acid and the kind of an acid-labile group (i.e., group unstable to an acid). Many reports have been published regarding examinations in which the kind of acid is changed in, for example, a resist of polyhydroxystyrene and a polyhydroxystyrene/(meth)acrylate copolymerization base for KrF lithography. For example, U.S. Pat. No. 5,744,537 discloses that an excellent pattern shape can be obtained when an acid generator that generates a camphorsulfonic acid is added. However, in a polymer for ArF having an alicyclic structure, acid-elimination reactivity is low, and the eliminatin reaction does not advance in the camphorsulfonic acid even if it is the same as polyhydroxystyrene and a polyhydroxystyrene/(meth)acrylate copolymer acid-elimination group. Herein, (meth)acrylate denotes methacrylate and/or acrylate.
On the anion side of the onium salt, a fluorinated alkylsulfonic acid having high acidity is chiefly applied. The fluorinated alkylsulfonic acid includes trifluoromethanesulfonic acid, a nonafluorobutanesulfonic acid and a hexadecafluorooctanesulfonic acid. Additionally, an arylsulfonic acid that has undergone fluorine substitution or fluorine alkyl substitution can be included. In greater detail, examples thereof include a 4-fluorobenzenesulfonic acid, a 3-benzenesulfonic acid, a 2-benzenesulfonic acid, a 2,4-difluorobenzenesulfonic acid, a 2,3-difluorobenzenesulfonic acid, a 3,4-difluorobenzenesulfonic acid, a 2,6-difluorobenzenesulfonic acid, a 3,5-difluorobenzenesulfonic acid, a 2,3,4-trifluorobenzenesulfonic acid, a 3,4,5-trifluorobenzenesulfonic acid, a 2,4,6-trifluorobenzenesulfonic acid, a 2,3,4,5,6 pentafluorobenzenesulfonic acid, a 4-trifluoromethylbenzenesulfonic acid, a 5-trifluoromethylbenzenesulfonic acid, a 6-trifluoromethylbenzenesulfonic acid, and a 4-trifluoromethylnaphthyl-2-sulfonic acid.
On the other hand, line-edge roughness and a dimensional difference (I/G bias) between a sparse pattern and a dense pattern have been to-be-solved matters correspondingly with the advancement of microfabrication. It is conventionally well known that a dimensional difference arises between a dense pattern and a sparse pattern after being developed even if they have the same dimensions on a mask. This problem is critical especially in a dimension exceeding a wavelength. The reason is because they differ in optical strength resulting from a difference in optical interference in the image formation of the dense pattern and the sparse pattern.
For example, FIG. 1 shows a pitch of a repetitive line of 0.18 microns, which is indicated by the horizontal axis, and a length of the line corresponding to the changing pitch, which is indicated by a vertical axis, under an optical condition that the wavelength is 248 nm, NA is 0.6, and σ is 0.75. The size of an optical image is temporarily thinned and becomes thicker as the pitch becomes larger when specifications are formed so that a line length becomes 0.18 microns in a 0.36-micron pitch (0.18-micron line and 0.18-micron space). The result of a resist line length calculated after being developed is also shown. The resist size and the optical-image size are shown by use of simulation software PROLITH2 Ver.6.0, which is sold by KLA-Tencor Corporation (former Finle Technologies Inc.). The resist size is thinned correspondingly with the enlargement of the pitch, and is further thinned correspondingly with an increase in acid diffusion.
The problem of sparseness/denseness dependence in which the size of the sparse pattern becomes thinner than that of the dense pattern has become a growing concern. It can be understood from the aforementioned simulation result that a method for reducing acid diffusion is effective as a method for reducing the sparseness/denseness dependence.
However, disadvantageously, ruggedness or surface roughening will occur because of a standing wave in a sidewall of a resist pattern after being developed, or line-edge roughness will become large if the acid diffusion is excessively reduced. For example, FIG. 2 shows a calculation result of a resist cross-sectional shape of a 0.18 μm-line-and-space pattern obtained when an acid-diffusion distance is changed on an Si base by use of the simulation software PROLITH Ver. 6.0 of KLA-Tencor Corporation.
It is shown that the ruggedness in the sidewall caused by the standing wave becomes prominent in proportion to a decrease in the acid diffusion distance. A similar tendency is also shown with regard to the line-edge roughness observed from above SEM. That is, the line-edge roughness increases in proportion to a reduction in the acid diffusion. A general method to reduce the line roughness is to increase the acid diffusion distance, but it is impossible to further improve sparseness/denseness dependence according to this method.
In FIG. 1, a dimensional difference between a dense pattern with a small pitch and a sparse pattern with a large pitch becomes large in proportion to an increase in the acid diffusion distance. That is, it is shown that the sparseness/denseness dependence becomes great. A reduction in the line-edge roughness and a reduction in the sparseness/denseness dependence stand in a trade-off relationship, and it can be considered that it is difficult for them to coexist easily.
A method for improving a light contrast can be mentioned as a method for improving line-edge roughness. For example, the line-edge roughness is lowered in proportion to an increase in line width if an exposure wavelength is the same. In the case of a repetitive pattern, off-axis illumination (e.g., annular illumination and quadrupole illumination) and a phase-shift mask have smaller line-edge roughness in proportion to an increase in NA of a stepper than normal illumination and a normal Cr mask, respectively, even if the exposure wavelength is the same, and even if the dimensions are the same. There is a correlation between the optical contrast of the line edge of a pattern and the line-edge roughness, and the line-edge roughness is reduced as the optical contrast of the line edge becomes steeper. In the exposure wavelength, shorter-wavelength exposure is expected to have smaller line-edge roughness. However, there is a report that ArF exposure should have a higher optical contrast and smaller line-edge roughness because of its shorter wavelength, although, in practice, KrF exposure is superior to ArF exposure in comparison to the line-edge roughness in KrF exposure and that in ArF exposure (SPIE 3999,264(2001)). This results from a performance difference between the resist material of KrF and that of ArF, and this indicates that line-edge roughness caused especially by the material in ArF exposure is a serious problem. Therefore, there is a demand to produce an acid generator that does not worsen sparseness/denseness dependence while improving line-edge roughness.